git-p4: change "commitish" typo to "committish"

This was the only occurrence of "commitish" in the tree, but as the
log will reveal we've had others in the past. Fixes up code added in
00ad6e3 ("git-p4: work with a detached head", 2015-11-21).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -2099,11 +2099,11 @@ def run(self, args):
 
         commits = []
         if self.master:
-            commitish = self.master
+            committish = self.master
         else:
-            commitish = 'HEAD'
+            committish = 'HEAD'
 
-        for line in read_pipe_lines(["git", "rev-list", "--no-merges", "%s..%s" % (self.origin, commitish)]):
+        for line in read_pipe_lines(["git", "rev-list", "--no-merges", "%s..%s" % (self.origin, committish)]):
             commits.append(line.strip())
         commits.reverse()